1. Field of the Invention
The present invention relates to a wiring configuration of a transformer used in a multi-output switching power supply.
2. Description of the Related Art
For example, in the liquid crystal television, a multi-output switching power supply for generating a plurality of voltages according to the voltage specification of each part of the television is incorporated. Generally, the multi-output switching power supply includes a rectifying circuit for rectifying an AC voltage, a transformer which primary winding is provided with the output of the rectifying circuit and from which secondary winding a plurality of voltages are taken out, a switching element connected to the primary winding of the transformer, a feedback circuit for feeding back a predetermined voltage taken out from the secondary winding, and a switching control circuit for controlling the operation of the switching element based on the output of the feedback circuit. Examples of such multi-output switching power supply are disclosed in patent article 1 and patent article 2 to be hereinafter described.
FIG. 4 is a schematic cross sectional view showing a wiring configuration of a conventional transformer 30 used in the multi-output switching power supply. Reference character 30a is a bobbin on which the wiring is winded, and a bias winding B, primary windings P1 and P2, and secondary windings S1 to Sn are winded on the bobbin 30a. The wiring configuration of the switching power supply transformer is disclosed in patent article 3 to be hereinafter described. The switching power supply of the present article is not of a multi-output type.
Patent article 1: Japanese Laid-Open Publication No. 2003-153530
Patent article 2: Japanese Laid-Open Publication No. 10-327581
Patent article 3: Japanese Laid-Open Publication No. 2000-184714
The multi-output switching power supply has a problem in that noise is produced at the primary winding since the voltage regulation in the primary winding is large, which noise influences the secondary winding. Thus, in a case of the transformer 30 shown in FIG. 4, each voltage output from the secondary winding S1 to Sn fluctuates and variation becomes large due to noise produced at the primary windings P1, P2. Further, the voltage induction by the leakage inductance tends to occur at the wiring with large output voltage out of the secondary windings S1 to Sn since the number of windings of the coil is large, which may cause excess output due to the voltage induction.
As a countermeasure thereof, a method of arranging a circuit for suppressing noise or inductive voltage on the primary side and the secondary side of the transformer is proposed, but this increases the number of circuit components and leads to increase in cost. Although the output voltage is desirably stabilized without arranging an extra circuit, an effective solution for the above problem is not described in patent article 1 to patent article 3. Patent article 1 discloses a technique for noise measure, but increase in cost is inevitable since a semiconductor element for short circuiting the secondary side of the transformer is essential. Patent article 2 also discloses a technique for suppressing noise or ripple, but the problem of increase in cost cannot be resolved since an additional wiring is required. Patent article 3 aims to eliminate lowering in bonding degree in time of low voltage input to enhance power conversion efficiency, and does not respond to the above problem. The present article also has a feature in the wiring configuration of the primary side, but does not have a feature in the wiring configuration on the secondary side as in the present invention to be hereinafter described.